Slices of meat, vegetable upon one (9)
I believe the answer is:
pepperoni
'slices of meat' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'vegetable upon one' is the wordplay.
'vegetable' becomes 'pepper' (pepper is a kind of vegetable).
'upon' becomes 'on' (I've seen this in another clue).
'one' becomes 'i' (Roman numeral).
'pepper'+'on'+'i'='PEPPERONI'
Can you help me to learn more?
(Other definitions for pepperoni that I've seen before include "Butterfly-attracting shrub" , "Seasoned beef/pork sausage" , "Spicy Italian sausage" , "something spicy" , "spicy food" .)